The present exemplary embodiment relates to miniaturized genetic, biochemical, and chemical processes related to analysis, synthesis, and purification procedures. More specifically, the present exemplary embodiment provides an apparatus and method for improved electrostatic merging and mixing of liquid droplets in which two such liquid droplets are moved towards each other. It finds particular application in conjunction with combinatorial chemistry and nanocalorimetry, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Existing electrostatic drop merger concepts are described in U.S. application Ser. No. 10/115,336, titled “Apparatus and Method for Using Electrostatic Force to Cause Fluid Movement”. Those designs, i.e. the single capacitor design, consist of two electrodes laid out on a single substrate. The substrate and the electrodes are covered with a dielectric substance which insulates the electrodes. The electrodes are arranged in a straight edge pattern as well as a triangle or chevron pattern, spaced apart, so that a gap is formed between the electrodes. A first droplet is deposited in an asymmetrical pattern across the gap between the electrodes such that a larger volume of the droplet rests on one of the electrodes. Another droplet is deposited in close proximity to the first droplet, but on the opposite side of the gap. When a voltage is applied across the electrodes, the first droplet moves towards a centering position across the gap, thus in an equilibrium position between the two electrodes, where it touches the second, stationary, droplet and the droplets merge together.
When two droplets of equivalent size are brought together by moving one droplet into another stationary droplet, the droplets coalesce into a single droplet. The two droplets touch each other such that one side of the combined droplet has the liquid from the first droplet and the other side of the combined droplet has the liquid from the second droplet. Mixing occurs primarily due to diffusion between the two liquids at the boundary between them.
Using the existing electrostatic drop merger designs of U.S. application Ser. No. 10/115,336, mixing time may be decreased to some extent by using droplets of different sizes. If the first droplet is smaller than the other stationary droplet, and the droplets are brought together, the momentum of the smaller droplet will cause a swirling motion in the combined droplet. This swirling motion both increases the internal area over which the diffusion occurs and, depending on relative speed, may create a shearing motion inside the combined droplet, a motion which may create internal weak vortices (packets of rotating fluid) which further enhance mixing rates. Additionally, the smaller droplet may be moved forcibly into the larger, stationary, droplet. However, as the smaller droplet's diameter (and hence its mass) decreases, the momentum (or kinetic energy) of the smaller droplet decreases as well, thus decreasing its ability to enhance mixing.
Another area of study directed to the movement of fluids is being undertaken at Duke University, Durham, N.C., under the paradigm of digital microfluidics, which is based upon micromanipulation of discrete droplets. Microfluidic processing is performed on unit-sized packets of fluid which are transported, stored, mixed, reacted or analyzed in a discrete manner using a standard set of basic construction.
Research has focused on the use of electrowetting arrays to demonstrate the digital microfluidic concept. Electrowetting is essentially the phenomenon whereby an electric field can modify the wetting behavior of a droplet in contact with an insulated electrode. If an electric field is applied non-uniformly, then a surface energy gradient is created which can be used to manipulate a droplet sandwiched between two plates.